In natural mineral oil deposits, mineral oil is present in the cavities of porous reservoir rocks which are sealed toward the surface of the earth by impervious top layers. The cavities may be very fine cavities, capillaries, pores or the like. Fine pore necks may, for example, have a diameter of only about 1 μm. As well as mineral oil, including fractions of natural gas, a deposit comprises water with a greater or lesser salt content.
In mineral oil production, a distinction is generally drawn between primary, secondary and tertiary production. In primary production, the mineral oil flows, after commencement of drilling of the deposit, of its own accord through the borehole to the surface owing to the autogenous pressure of the deposit.
After primary production, secondary production is therefore used. In secondary production, in addition to the boreholes which serve for the production of the mineral oil, the so-called production bores, further boreholes are drilled into the mineral oil-bearing formation. Water is injected into the deposit through these so-called injection bores in order to maintain the pressure or to increase it again. As a result of the injection of the water, the mineral oil is forced slowly through the cavities into the formation, proceeding from the injection bore in the direction of the production bore. However, this only works for as long as the cavities are completely filled with oil and the more viscous oil is pushed onward by the water. As soon as the mobile water breaks through cavities, it flows on the path of least resistance from this time, i.e. through the channel formed, and no longer pushes the oil onward.
By means of primary and secondary production, generally only approx. 30 to 35% of the amount of mineral oil present in the deposit can be produced.
It is known that the mineral oil yield can be enhanced further by measures for tertiary oil production. A review of tertiary oil production can be found, for example, in “Journal of Petroleum Science of Engineering 19 (1998)”, pages 265 to 280. Tertiary oil production includes, for example, thermal methods in which hot water or steam is injected into the deposit. This lowers the viscosity of the oil. The flow medium used may likewise be gases such as CO2 or nitrogen.
Tertiary mineral oil production also includes methods in which suitable chemicals are used as assistants for oil production. These can be used to influence the situation toward the end of the water flow and as a result also to produce mineral oil hitherto held firmly within the rock formation.
Viscous and capillary forces act on the mineral oil which is trapped in the pores of the deposit rock toward the end of the secondary production, the ratio of these two forces relative to one another being determined by the microscopic oil separation. By means of a dimensionless parameter, the so-called capillary number, the action of these forces is described. It is the ratio of the viscosity forces (velocity x viscosity of the forcing phase) to the capillary forces (interfacial tension between oil and water x wetting of the rock):
      N    c    =                    μ        ⁢                                  ⁢        v                    σ        ⁢                                  ⁢        cos        ⁢                                  ⁢        θ              .  
In this formula, μ is the viscosity of the fluid mobilizing mineral oil, ν is the Darcy velocity (flow per unit area), σ is the interfacial tension between liquid mobilizing mineral oil and mineral oil, and θ is the contact angle between mineral oil and the rock (C. Melrose, C. F. Brandner, J. Canadian Petr. Techn. 58, October-December, 1974). The higher the capillary number, the greater the mobilization of the oil and hence also the degree of oil removal.
It is known that the capillary number toward the end of secondary mineral oil production is in the region of about 10−6 and that it is necessary to increase the capillary number to about 10−3 to 10−2 in order to be able to mobilize additional mineral oil.
For this purpose, it is possible to conduct a particular form of the flooding method—what is known as Winsor type III microemulsion flooding. In Winsor type III microemulsion flooding, the injected surfactants should form a Winsor type III microemulsion with the water phase and oil phase present in the deposit. A Winsor type III microemulsion is not an emulsion with particularly small droplets, but rather a thermodynamically stable, liquid mixture of water, oil and surfactants. The three advantages thereof are that                a very low interfacial tension a between mineral oil and aqueous phase is thus achieved,        it generally has a very low viscosity and as a result is not trapped in a porous matrix,        it forms with even the smallest energy inputs and can remain stable over an infinitely long period (conventional emulsions, in contrast, require high shear forces which predominantly do not occur in the reservoir, and are merely kinetically stabilized).        
The Winsor type III microemulsion is in an equilibrium with excess water and excess oil. Under these conditions of microemulsion formation, the surfactants cover the oil-water interface and lower the interfacial tension a more preferably to values of <10−2 mN/m (ultra-low interfacial tension). In order to achieve an optimal result, the proportion of the microemulsion in the water-microemulsion-oil system, with a defined amount of surfactant, should by its nature be at a maximum, since this allows lower interfacial tensions to be achieved.
In this manner, it is possible to alter the form of the oil droplets (interfacial tension between oil and water is lowered to such a degree that the smallest interface state is no longer favored and the spherical form is no longer preferred), and they can be forced through the capillary openings by the flooding water.
When all oil-water interfaces are covered with surfactant, in the presence of an excess amount of surfactant, the Winsor type III microemulsion forms. It thus constitutes a reservoir for surfactants which cause a very low interfacial tension between oil phase and water phase. By virtue of the Winsor type III microemulsion being of low viscosity, it also migrates through the porous deposit rock in the flooding process (emulsions, in contrast, can become trapped in the porous matrix and block deposits). When the Winsor type III microemulsion meets an oil-water interface as yet uncovered with surfactant, the surfactant from the microemulsion can significantly lower the interfacial tension of this new interface, and lead to mobilization of the oil (for example by deformation of the oil droplets).
The oil droplets can subsequently combine to a continuous oil bank. This has two advantages:
Firstly, as the continuous oil bank advances through new porous rock, the oil droplets present there can coalesce with the bank.
Moreover, the combination of the oil droplets to give an oil bank significantly reduces the oil-water interface and hence surfactant no longer required is released again. Thereafter, the surfactant released, as described above, can mobilize oil droplets remaining in the formation.
Winsor type III microemulsion flooding is consequently an exceptionally efficient process, and requires much less surfactant compared to an emulsion flooding process. In microemulsion flooding, the surfactants are typically optionally injected together with co-solvents and/or basic salts (optionally in the presence of chelating agents). Subsequently, a solution of thickened polymer is injected for mobility control. A further variant is the injection of a mixture of thickening polymer and surfactants, co-solvents and/or basic salts (optionally with chelating agent), and then a solution of thickening polymer for mobility control. These solutions should generally be clear in order to prevent blockages of the reservoir.
The requirements on surfactants for tertiary mineral oil production differ significantly from requirements on surfactants for other applications: suitable surfactants for tertiary oil production should reduce the interfacial tension between water and oil (typically approx. 20 mN/m) to particularly low values of less than 10−2 mN/m in order to enable sufficient mobilization of the mineral oil. This has to be done at the customary deposit temperatures of approx. 15° C. to 130° C. and in the presence of water of high salt contents, more particularly also in the presence of high proportions of calcium and/or magnesium ions; the surfactants thus also have to be soluble in deposit water with a high salt content.
To fulfill these requirements, there have already been frequent proposals of mixtures of surfactants, especially mixtures of anionic and nonionic surfactants.
U.S. Pat. No. 7,119,125 B1 describes a mixture of sulfated Guerbet alcohol alkoxylate and of low molecular weight sulfated alkyl alkoxylate in oil production. Particularly good emulsifying properties are attributed to the bimodal distribution. However, these emulsifying properties are of no great importance in Winsor type III microemulsion flooding. Too much surfactant would be required for the emulsification of oil, and the shear forces required are barely present in the flooding operation (apart from the region around the injector).
US-A 2008/217064 describes a drilling fluid solution comprising a nonionic surfactant—consisting of at least one branched alkyl ethoxylate and an end group-capped alkyl ethoxylate—and also a detergent builder and a viscosifier. The nonionic surfactant may be a C10 Guerbet alcohol ethoxylate.
US-A 2009/270281 describes the use of a surfactant mixture for mineral oil production, which comprises at least one surfactant having an alkyl radical of 12 to 30 carbon atoms and a branched cosurfactant having an alkyl radical of 6 to 11 carbon atoms. The degree of branching of the alkyl radical in the cosurfactant ranges from 1 to 2.5, and can thus comprise Guerbet alcohols of the 2-ethylhexyl or 2-propylheptyl type. The cosurfactants may be alcohol ethoxylates or anionically modified alcohol ethoxylates (e.g. alkyl ether sulfate).
The use parameters, for example type, concentration and mixing ratio of the surfactants used with respect to one another, are therefore adjusted by the person skilled in the art according to the conditions existing in a given oil formation (for example temperature and salt content).
As described above, mineral oil production is proportional to the capillary number. The lower the interfacial tension between oil and water, the higher it is. The higher the mean number of carbon atoms in the crude oil, the more difficult it is to achieve low interfacial tension. Suitable surfactants for low interfacial tensions are those which possess a long alkyl radical. The longer the alkyl radical, the better it is possible to reduce the interfacial tensions. However, the availability of such compounds is very limited.